Ferrets are true carnivores and cannot handle a diet containing more than 4% fiber. There are several good commercial ferret foods available that are dry foods.
Guinea pigs, like rabbits, are true herbivores, although the physiology and function of their gastrointestinal system is less well understood than that of the rabbit, which has been well investigated.
Rabbits are herbivores and are considered nibblers, in that they eat continuously. They have complex digestive systems and are very efficient at processing food. They also have very specific dietary needs. If you introduce new foods too quickly, or feed inappropriate food choices, the rabbit’s normal digestive flora (normal bacteria) will be disturbed and may lead to a sick rabbit.